PDT is a medical treatment that utilizes light activated photosensitive dyes to elicit a beneficial biological response. These dyes, or photosensitizers, elicit a biological response when irradiated with light within a certain wavelength range but are inert without such illumination.
A promising area for the application of PDT is in the treatment of cardiovascular disease indications such as vulnerable plaque, atherosclerosis and restenosis. Such applications generally involve the delivery of photosensitizers into a blood vessel to be treated, followed by the delivery of light to the target tissue, often through a light delivery catheter. One of the challenges associated with PDT and other treatments employing endovascular light delivery arises from the tendency of blood to attenuate light in the treatment region. The PDT effect can be significantly degraded if blood is not eliminated from the light path. This adverse effect exists for PDT methods utilizing all wavelengths of light treatment, but is particularly significant for treatments using short wavelengths of light (e.g., less than 610 nm), where light attenuation by blood is most significant.
Balloon catheters have been used in an effort to eliminate blood during the light delivery process. A commonly employed approach is to place a light-emitting fiber within a transparent or translucent angioplasty-style balloon. Blood is removed from the light treatment region by inflating the balloon to displace the blood. This approach has been employed almost exclusively for most cardiovascular PDT methods.
For example, Spears, U.S. Pat. No. 4,512,762, discloses a balloon catheter equipped with flexible optical fibers for transmission of light from an external source for illumination of the interior of the inflated balloon. By inflation of the balloon, the blood between the balloon and the diseased vascular wall is displaced. Other examples of PDT catheters employing an optical element within a displacement balloon include the following: Narciso, U.S. Pat. Nos. 5,169,395; 5,441,497; 5,700,243; Leone, U.S. Pat. Nos. 5,797,868; 5,891,082; EP 0 732 085; EP 0 732 079; Ligtenberg et al., EP 0 732 079 A1; Bower et al., U.S. Pat. Nos. 6,013,053; 6,086,558; Overholt et al., U.S. Pat. No. 6,146,409; Aita et al., U.S. Pat. No. 6,132,423; and Amplatz et al., U.S. Pat. No. 5,620,438.
A significant problem with the displacement balloon approach described in the foregoing patents is that it fails to fully displace the blood, leaving some blood trapped between the outside surface of the inflated balloon and the inside surface of the vascular wall. With this approach, adequate displacement generally requires a balloon that is at least as long as the light treatment zone. To achieve a cylindrical shape that is consistent with the shape of the vessel being treated, non-compliant balloon materials are typically used, because balloons made from such materials retain their shape when inflated. To avoid injuries associated with mechanical trauma due to inflation of a non-compliant cylindrically-shaped balloon, it is necessary to inflate the balloon using a very low pressure. However, such a balloon inflated at low pressure usually cannot exert sufficient force to adequately displace the surrounding blood. Consequently, the angioplasty balloon can at best be a compromise solution since under-inflation prevents mechanical trauma without achieving adequate blood removal, whereas over-inflation achieves better blood removal but with increased risk of mechanical trauma.
One attempt to overcome this shortcoming of the angioplasty design approach is to utilize a so-called “weeping balloon” as described in Kume et al., U.S. Pat. No. 5,876,426; Leone, U.S. Pat. No. 5,709,653; and Amplatz et al., U.S. Pat. Nos. 5,833,68 and 5,964,751. Each of these patents discloses a light delivery catheter fitted with a porous angioplasty-style balloon that leaks fluid from its surface to flush blood from around the periphery of the balloon. Though such weeping balloons may provide better blood elimination than standard angioplasty-style balloons, they suffer from some significant shortcomings. For example, flushing fluid delivered in such a manner tends to find the path of least resistance to escape into the open blood vessel, leaving pockets of blood trapped between the balloon and the vessel wall. Another limitation is that when the balloon is deflated blood can be sucked into the balloon where it will attenuate light delivered in subsequent inflation and treatment cycles.
Other shortcomings of both the standard displacement balloon and the weeping balloon approaches can arise from the use of non-compliant balloon materials. Each of the patents referenced above generally discloses a light delivery catheter having an elongated, tubular balloon that extends along the length of the catheter at least as long as the length of the light treatment zone. As explained by Saab, in “Applications of High-Pressure Balloons in the Medical Device Industry,” published in Medical Device and Diagnostic Industry, September 2000, pg. 86, achieving this tubular shape in an inflated balloon generally requires the use of a relatively non-compliant (i.e., less elastic) balloon material that will retain its shape when inflated. Because non-compliant balloons are more rigid and do not conform to the shape of the vessel, such balloons have a greater tendency to cause mechanical trauma to the vessel. The resulting injury response can lead to restenosis.
The non-compliant angioplasty balloon is also limited in its ability to treat long tortuous vessels. When using a light emitting element within an angioplasty balloon it is necessary to fully inflate the balloon to adequately displace blood. However, this can be difficult in tortuous vessels, especially if the treatment length is greater than 1-2 cm. This is due to the fact that, when inflated, the non-compliant angioplasty balloon tends to inflate in a straight line, rather than follow the curvature of the vessel. The result is that the balloon tends to straighten the vessel, causing mechanical trauma to the vessel.
The non-compliant angioplasty balloon also has a limited ability to treat small diameter vessels. When using the angioplasty balloon approach, it is necessary to mount the non-compliant angioplasty balloon on the catheter shaft overlapping the light treatment zone. However, use of such a non-compliant balloon adds to the diameter of the device in the treatment zone, thereby limiting access to small vessel diameters.
Still another shortcoming of the non-compliant angioplasty balloon is in the treatment of vessels whose diameter tapers or otherwise changes within the length of the section to be treated. When using an angioplasty style balloon it is necessary to inflate the balloon to displace the blood. This can cause injury within the smaller diameter regions of the vessel being treated since these balloons generally have a constant diameter along their length.
Furthermore, non-compliant balloon devices have a limited ability to treat multiple vessel diameters with a single device. With the angioplasty balloon approaches, the device generally must be correctly sized to the vessel to be treated. This requires that a significant stock of devices be kept on hand and also limits the various vessel geometries that can be treated.
Thus, there is a significant need for improved light delivery catheters that can provide improved blood elimination along the light treatment region and can avoid the shortcomings associated with prior devices employing non-complaint angioplasty balloons. The present invention provides improved light delivery catheters having these and other features and advantages.